The crucial idea that Michael Sandel works toward at the end of Justice is that we have obligations that extend beyond the ends we have chosen: we have obligations of solidarity. Communitarians, like Sandel, argue that we have obligations to our several communities because we are members of them, shaped by them, sharing their lives.
His most powerful case, I think, is our obligations of patriotism. We have a strong obligation to our country. I can imagine circumstances in which someone would have to renounce that obligation to serve another, higher obligation. But the cases in which people actually do renounce their citizenship on principle are extraordinarily rare.
I think marriage is a solidarity that we choose. When we choose it, though, it creates an obligation of solidarity that is like our obligation to our country. It is a deep, enduring obligation. It can, in principle, be cast off, but only for the rarest and most compelling of reasons.